(a) Field of the Invention
The present invention relates to Indium Tin Oxide (ITO) fine powder for forming a transparent, conductive, thin film mainly used for, for instance, liquid crystal display and electromagnetic wave-shielding and a method for preparing the ITO fine powder.
(b) Description of the Prior Art
A transparent, conductive, thin film has been a necessary and indispensable structural element in display elements such as LCD (liquid crystal display) elements, irrespective of active matrix type or simple matrix type, with the wide spread of electronic equipment. There has widely been used an ITO material as a principal material for forming such a thin film and accordingly, it is a matter of course that there has been desired for the development or improvement of ITO powdery materials as well as various thin film-forming techniques.
As techniques for forming transparent, conductive, thin films, there have been known, for instance, vacuum evaporation, sputtering, CVD and spray methods, but these techniques suffer from various problems in that they are insufficient in productivity, that they require the use of expensive devices, that they cost too much for forming thick films and that if the resulting films are thin, then the thin films are insufficient in durability. For this reason, there have recently been investigated methods comprising coating a paint and varnish in which ITO fine powder is dispersed, in place of these techniques and ITO fine powder suitably used for these coating methods has likewise been variously investigated.
Regarding the crystal structure, ITO has been known to have a bixbyite type or corundum type crystal structure, but those industrially put into practical use are overwhelmingly bixbyite type ones. This is because, the typical method for producing the ITO fine powder which has been practically used and permits the mass-production thereof is a wet method comprising the steps of reacting an aqueous solution containing an indium salt and a tin salt with an alkali to thus co-precipitate hydroxides of indium and tin and then decomposing the hydroxides with heating and accordingly, most of the ITO fine powder products prepared by this method are bixbyite type ones.
This bixbyite type ITO satisfies some requirements for ITO fine powder suitable for forming a transparent, conductive, thin film, such as low powder flow resistance and high dispersibility, but it has a low true specific gravity and a low green density and, what is more, the Seebeck coefficient (proportional to the reciprocal number of carrier electron density) of the ITO powder as an indication of the conductivity thereof is not always stable at ordinary temperature and even during heating. The true specific gravity and green density of the ITO fine powder serve as an indication relating to the packing characteristics of the powder in a film when the ITO fine powder is incorporated into paints and varnishes and the resulting paints and varnishes are formed into transparent, conductive, thin films. In this connection, the higher the true specific gravity and green density of the ITO fine powder, the lower the electric resistance of the resulting transparent, conductive, thin film. In addition, the temperature-stability (or independency) of the Seebeck coefficient of the powder is very important to ensure the thermal resistance thereof which is required when heating the paint and varnish at around 200.degree. C. in the process for forming a transparent, conductive, thin film.